TAMPA, Fla. (WFLA) — Drugmaker Moderna says it has seen its COVID-19 vaccine’s antibody levels decline over time and believes people who received two doses of it will need a booster shot before winter to protect against virus variants.

“We believe that increased force of infection resulting from Delta, non-pharmaceutical intervention (NPI) fatigue, and seasonal effects (moving indoors) will lead to an increase of breakthrough infections in vaccinated individuals,” the company said in its second-quarter earnings presentation Thursday.

According to the presentation, Moderna’s COVID-19 shot was about 93% effective through six months after the second dose, but antibody levels were expected to wane and impact vaccine efficacy.

“Given this intersection, we believe dose 3 booster will likely be necessary prior to the winter season,” the presentation said.

The COVID-19 vaccine is Moderna’s only commercially approved product. It also is developing several vaccines that aim to guard against the flu, Zika and HIV among other viruses. Those are all in early stages of clinical testing, according to its website.

The company also is testing a potential booster shot for the COVID-19 vaccine.

Moderna’s COVID-19 vaccine brought in more than $4 billion in second-quarter sales, helping to push the vaccine developer into a profit.

Moderna’s announcement comes after pharmaceutical giant Pfizer Inc. said its COVID-19 vaccine remained effective months after the second dose and had become a top seller. It brought in nearly half the company’s revenue — $7.84 billion from direct sales and revenue split with its partner, Germany’s BioNTech.

Moderna’s COVID-19 vaccine has received emergency authorization for use in more than 50 countries.

The Associated Press contributed to this report.